It has long been known that the heart muscle provides its pumping function in response to electrical events which occur in the atrium and ventricle of the heart. The heart is structured such that conductive tissue connects the atrium and the ventricle and provides a path for the conduction of electrical signals between the two areas. In the operation of a normal heart, a natural atrial event spontaneously occurs in the atrium and a corresponding ventricular event occurs later in the ventricle after a time interval that is typically denoted the AV interval. After the natural occurrence of the ventricular event a new atrial event naturally occurs in the atrium to trigger a succeeding ventricular event. The synchronized electrical events occurring in the atrium and ventricle cause the heart muscle to rhythmically expand and contract and thereby pump blood throughout the body.
In a diseased heart atrial and ventricular events do no naturally occur in the required synchronized manner and the pumping action of the heart is therefore irregular and ineffective to provide the required circulation of blood within the body. The required synchronized activity of such diseased hearts can be maintained by implanting a pacemaker device which applies synchronized stimulating voltage signals to either or both of the atrium and ventricle to pace the heart.
In the early stages of pacemaker development pacemakers were employed to asynchronously stimulate the ventricle of the heart without regard to natural electrical activity occurring in either the atrium or the ventricle. Although this approach had the advantage of simplicity, there was considerable risk due to the fact that paced ventricular events could interact with natural ventricular events to cause the heart to go into a dangerous fibrillation.
As the art of pacing advanced, pacemakers were provided with circuitry which sensed the occurrence of natural ventricular and atrial activity and paced the heart in either the atrium or ventricle only when required to maintain proper operation of the heart.
At the present time it is deemed desirable in some cases to operate a dual chamber pacer in what is known as the DDD mode, wherein electrical events are sensed in the atrium and ventricle and the atrium and ventricle are paced accordingly. Pacers may also be operated in the VDD mode to sense electrical events in the atrium and ventricle and to pace the ventricle. Other pacer modes of operation are employed to sense or pace in either the atrium or ventricle, as required for the particular needs of a heart.
It has been found that pacemakers which operate in the DDD or VDD modes can, under certain circumstances, sustain a dangerous tachycardia condition. A pacer sustained tachycardia condition is defined as an operational pacing state wherein the pacer erroneously stimulates the ventricle of a heart at a dangerously high rate for sustained periods of time.
Pacer sustained tachycardia is initiated when a ventricular event occurs at a time during which the connective tissue between the atrium and ventricle can transmit retrograde electrical signals from the ventricle to the atrium. The conduction of the ventricular signal to the atrium provides a spurious electrical signal in the atrium which appears to the pacer to be a natural atrial event. The pacer senses this spurious retrograde atrial signal and then paces the ventricle at a programmed AV time period following the signal. The paced ventricular signal is conducted to the atrium where it is again erroneously detected by the pacer as a natural atrial event. The pacer therefore continues to pace the ventricle at a relatively high rate defined by the sum of the programmed AV interval and the retrograde conduction time between the ventricle and atrium. This high rate is sustained indefinitely by the pacer, because retrograde conduction ensures that the pacer detects what appear to be high rate atrial events and tracks these spurious atrial events by generating corresponding high rate ventricular paces. This pacer sustained tachycardia condition overstimulates the heart, at considerable danger to the patient.
It is therefore an object of the invention to provide a pacemaker which will operate in a manner that avoids pacer sustained tachycardia.
It is a further object of the invention to provide such a pacemaker that will have a means for breaking out of any pacer sustained tachycardia that occurs.